The present invention relates to a method of forming an article via injection of plastics material into a mould.
It is known to expand a mould containing foamable molten plastics material. For instance, in U.S. Pat. No. 3,793,415, to A. G. Smith, assigned to ICI and applied for on Nov. 16, 1971, formation of articles having a foamed core is described. A foamable composition is injected into a mould to fill it without foaming. A skin forms against the inside surface of the mould. Movable inserts in the mould are retracted. This allows areas of the mould to foam. Alternatively, these areas are filled without foaming and the volume of the mould is enlarged by partial opening of the mould to allow the core of the moulding to foam. A variety of possible blowing agents is set out. This method is hereinafter referred to as xe2x80x9cthe ICI Methodxe2x80x9d.
A more complex method is the TAF method, as described in Japanese patent No. 51-27266 to Ashani Dow Limited and applied for on Jul. 1, 1972. It addresses the then prevalent problem of exothermic blowing agents, namely that during injection of them with the plastics material into the mould, gas bubblesxe2x80x94in practice nitrogen since diazobonamide was the usual blowing agentxe2x80x94form close to the surface of the material and burst through the surface before it had frozen off against the mould. The TAF method addresses this by pressurising the mould with nitrogen so as to prevent foaming until the mould has been filled and a skin has formed against the inside surface of the mould. After injection the gas is vented, allowing foaming to start. The mould volume is expanded and the material foams to fill it. The advantage was blemish-free surfaces of the final product.
A similar process is described in British patent application No. 2,010,168 by USM Corporation, applied for on Dec. 12, 1978. Again a skin is formed against the inside surface of the mould.
In my International Application No. PCT/GB/96/01706, now published under No WO 87/03800, I described and claimed (as proposed to be amended):
A method of forming an article via injection of plastics material into a mould, the finish formed article having thin wall portion(s) and thick wall portion(s), the thick wall portion(s) being at least partially foamed, the method consisting in the steps of:
providing a mould tool defining in its closed state, between its cavity part and its core part, narrow gap portion(s) whose mould part gap is to be substantially reproduced in the thin wall portion(s) of the article and wide gap portion(s) whose mould part gap is less than the thickness of the thick wall portion(s) of the finish formed article;
closing the mould tool to define the narrow and wide gap portions;
injecting a plastics material mixture comprising a basic polymer and a foam producing additive into the mould tool;
allowing the plastics material mixture to at least substantially solidify in the narrow gap portions of the mould tool to produce the thin wall portions of the finish formed article;
withdrawing at least a portion of one part of the mould tool from the other part before the plastics material mixture has at least substantially solidified in the wide gap portion(s) of the mould tool to allow the mixture to expand by foaming and form at least some of the thick wall portion(s) of the finish formed article; and
ejecting the article from the mould tool.
This method is hereinafter referred to as xe2x80x9cMy Original Methodxe2x80x9d.
It is possible to work My Original Method in conjunction with the ICI Method. Indeed in my International Application No. PCT/GB96/01706 on My Original Method, there are described alternatives where movable mould parts are drawn back to allow local foaming from thick wall portions.
However these alternatives were not expected to produce finish formed articles with a foam structure different from that otherwise obtained in My Original Method, that is to say with a clearly perceptible skin and a foam structure, resembling that of conventional foam.
In developing My Original Method, with a view to making larger foamed sections, I designed the tooling described in respect of the first embodiment below and experienced the very surprising xe2x80x9crapid expansion foamingxe2x80x9d, by withdrawing the movable part very early in the moulding cycle.
According to my present invention there is provided a method of forming an article via injection of plastics material into a mould, the method consisting in the steps of:
providing a mould tool defining in its closed state a mould cavity, the mould tool being adapted to be partially expandable from an injection volume for receiving a load of plastics material to a final, larger volume determining the finish formed shape of the article, the mould tool having at least one movable member, which is adapted to be withdrawn to increase the volume of the mould cavity, the difference in the injection and final volumes being represented by a void exposed in the mould cavity when the movable member is withdrawn,
closing the mould tool to delimit the injection volume,
injecting into the injection volume a plastics material mixture comprising a basic polymer and a foam producing additive,
withdrawing the movable member to allow the plastics material to take up the shape of the final volume and
opening the mould and ejecting the finish formed article;
wherein the withdrawal is effected with substantially minimal delay on filling of the injection space, whereby rapid expansion foaming occurs into the void on the withdrawal.
The actual delay after which rapid expansion foaming will not occur will depend upon the individual article being formed and details of the moulding tool. Too long a delay will result in a coherent skin being formed against the movable member, which inhibits the foaming from occurring throughout the final volume of the mould cavity. It is possible that a thin skin, which ruptures and loses coherence on foaming, may be allowed to form against the movable member.
In terms of conventional injection moulding practice, wherein the injection phase of the moulding machine cycle comprises an injection/filing stages followed by a packing/holding stage, the holding stage is minimised, if not dispensed with entirely. The delay between the filling stage and withdrawal of the movable member need only be sufficiently long to allow closure of a shut-off valve between the injection barrel and the moulding tool and/or sufficiently long to allow solidification of the plastics material in the gate of the moulding tool to prevent return flow of the material out of the tool. Therefore the movable member can be withdrawn immediately after the shut-off or gate solidification.
In making a small article, such as a cosmetics cream jar, I draw back the movable member within less than a second after filling of the mould, i.e. after the injection screw has ceased forwards movement. I have withdrawn the movable member within half a second of filling. Typically I can withdraw within one or two tenths of a second of filling; and I envisage that I could withdrawn without any delay, with a shut-off valve being closed at the same time as withdrawal. I would not expect to experience the rapid expansion foaming if the withdrawal was delayed for appreciably more than a second, at least without the movable member being insulated. For larger mouldings, it may be that longer delays can be tolerated due to the thermal mass of molten plastics material.
It is my present understanding that the foam forms spontaneously on withdrawal, with the movable member exposing molten plastics material to the void. To encourage this, I provide that the xe2x80x9coriginal thicknessxe2x80x9d dimension, from the surface of the movable member through the material to the portion of the mould tool on the opposite side of it, sufficiently large for the body of the material to retain its heats and for little cooling effect to be experienced at the movable member. This does not necessarily mean that this original, expanding thickness of the material cannot be thin in comparison with the thickness of the foam. I anticipate that it is possible to expand to at least the order of 5 times of the expanding thickness. Lesser expansion, for instance of the order of twice the expanding thickness is readily achievable. The expanding thickness can be of the same order of thickness as that of thin wall sections of other portions of the article which are not foamed.
Where, as described below, the foaming is against another portion of the article, which is not to be foamed, it should be thinner than the expanding thickness so that is solidified when the movable member is withdrawn from it. Typically this can be a third of the thickness of the expanding thickness.
This expanding thickness can be as thin as that of thin wall sections of the article which solidify before mould opening and without foamingxe2x80x94as taught in My Original Method. To further avoid the formation of a skin against the movable member, and to avoid stiction of it with the material, the surface of the movable member can be treated with a low friction material which inhibits conduction of heat from the melt to the member. Indeed, the member can advantageously be of material having less conductivity that metal, for example a ceramic material. Nevertheless, I do not rule out that a thin skin may form against the movable member, in for instance large mouldings taking a long time to fill.
I do not fully understand the rapid expansion foaming. It seems to be akin to the foaming which occurs in the mouth of a bottle of carbonated drink on uncontrolled release of its closure. Were it to be possible to measure this, I would expect the plastics material to be still liquid in the injection volumexe2x80x94or at least a portion of it against the movable memberxe2x80x94when the movable member is withdrawn immediately on filling of the mould. Further I believe that foaming occurs spontaneously on withdrawal, filling the void without delay.
The rapid expansion foaming results in a closed cell structure in the finished article with surprising rigidity. I believe the cell structure to result from the combination of:
rapid expansion of gas bubbles forming the cells.
rapid stretching of the cell walls due to the rapid expansion, and
rapid cooling of the cell walls due to the rapid adiabatic expansion of the gas bubbles.
The adiabatic expansion cools the gas, which acts endothermically to absorb heat from the cell walls. It may be that the cell walls solidify by this process, i.e. the latent heat of solidification being transferred to the endothermically expanding gas bubbles. Even if this effect does not completely solidify the foam, it significantly reduces the cooling time by comparison with that for a solid moulding of the same dimensions
The most surprising feature of the rapid expansion foaming is the rigidity of the resultant cell structure. I believe this to result from the polymer molecules being straightened in the direction of stretching of the cell walls, with the result that the polymer chains are substantially multi-directionally oriented and locally parallel to the cell walls.
Naturally, the foamed region has a low density, which is to be expected. However, the surprising rigidity exhibits itself in the cell structure being much more rigid than foamed materials of comparable density. For instance I have formed an article of foamed polypropylene, which appears on handling to have a density of the same order as that of expanded polystyrene. Normally expanded polystyrene can be compressed to a marked extent between thumb and forefinger. No give at all is perceptible in my article. Indeed the material can barely be indented by thumb nail. The foamed material appears to be nearly as rigid as other portions of the article which are moulded solid.
The foam producing additive essentially comprises gas, such as carbon dioxide, in solution in the plastics material mixture. The gas may be introduced by use of a blowing agent or directly as described in my UK patent applications Nos. 97029771.1 and 9706682.3, respectively dated Feb. 13 and Apr. 2, 1997. These are still unpublished. However, the mechanism for producing a solution of gas in the plastics material mixture forms not part of this invention, which is concerned with production of articles using such solution.
Where the mould tool merely comprises two parts, which are so arranged as to be able to be partially opened without exposing the mould cavity, typically by being formed in the manner of a piston and cylinder; the withdrawal is effected by such partial opening, which creates the void in the mould cavity. I would expect the majority of mould tools to include an additional movable part, which extends into the mould cavity and is pulled back on withdrawal to create the void in the mould cavity.
The movable member can be arranged to be pulled back in the direction in which the foaming will occur. Alternatively, the movable member can be arranged to be pulled back transversely of the predominant foaming direction. Typically, when the movable member is an additional part, it will be a sleeve arranged to be withdrawn longitudinally, exposing a circumferential void into which the foaming occurs radially outwards. Also, I envisage that the foaming may occur radially inwards. It should be understood that where a direction of foaming is specified, it is the general direction of foaming into the void which is referred to as opposed to the precise direction movement of the individual bubbles.
It will normally be convenient to withdraw the additional movable part transversely of the mould split plane, conveniently with a hydraulic or pneumatic ram.
Particularly where the movable member is an additional movable part, it is conveniently arranged to be moved forward again on mould opening for eject of the finish formed article from the mould tool. This can be achieved both where the member is withdrawn in the direction of foaming and where the foaming is transverse of the direction of withdrawal.
Where the direction of foaming is transverse of the direction of withdrawal, the foaming can be against a mould face which was completely obscured by the additional movable part from the injection volume on injection. In this arrangement, the melt foams across the void and impinges on the face of the mould tool. Normally this will result in a virtually skinless surface; in that the cell walls at the mould face coalesce to form a continuous surface, by the skin has no appreciable depth and there is no appreciable gradation in cell size close to the skinxe2x80x94although the cells may amalgamate locally throughout the body of the foam. In this arrangement, there are two alternatives, either the additional movable part when advanced has no appreciable gap between itself and the face of the mould, whereby the foam crossed the void vacated by the part only; or alternatively, there is a gap between the movable part and the face and the foam crosses both the void and this gap.
In an alternative, the additional movable part does not obscure the gap beyond itself from the injection volume, whereby plastics material is injected at both sides of the movable member. According to the thickness of the injected material at the respective sides and the extent to which they are skinned, one will foam towards the other or both will foam towards each other. Preferably, the movable member will be insulated and/or friction coated to such extend and/or withdrawn sufficiently soon that the foam(s) will adhere to each other or the non-foamed material to which the foamed material expands.
Particularly where the movable part is a sleeve, it will normally be positioned for the foaming expansion to be radially outwards. This has particular advantage where higher material density is required internally, such as in a container requiring a closely defined inside or in a xe2x80x9ccorkxe2x80x9d requiring a core to enable pulling with a cork screw.
Where the article is a structural item such as an automotive bumper or a building construction lintel, the movable part will preferably include at least one bar withdrawable to allow foaming to form a beam on a side of the article, which is not visible in use; whilst the opposite side is moulded without foaming to produce an appreciable skin with a visual surface.
Wherein the movable member extends within the space defining the finished formed shape and size of the article.
In designing the tooling described in respect of the first embodiment below, I invented another use for it, in that it can be used for injection blow moulding.
In conventional injection blow moulding, an injection moulded parason is formed as a first stage in one mould. The moulding is then indexed to a second cavity in the same machine, where it is inflated to the shape of the second mould by blowing gas into it. Alternatively, the parasons are transported in bulk to a bottling plant for instance where they are placed in a blowing machine, in which they are individually heated and then blown to shape in a mould. Both of these methods involve the use of a first cavity for moulding the parason and a separate second cavity for blowing of the parason.
In the injection blow aspect of my present invention, the parason is formed and a movable member is withdrawn to expose an extension to the cavity into which the parason is blown by gas pressure.
Thus according to second aspect of my present invention there is provided a method of forming an article via injection of plastics material into a mould, the method consisting in the steps of:
providing a mould tool defining in its closed state a mould cavity, the mould tool having a core part, a cavity part and an additional movable part whereby it is adapted to be partially expandable from an injection volume for receiving a load of plastics material to a final, larger volume determining the finish formed shape of the article, the additional movable part being adapted to be withdrawn to increase the volume of the mould cavity, the difference in the injection and final volumes being represented by a void exposed in the mould cavity when the movable part is withdrawn, the arrangement being such that there is a gap between the additional movable part when advanced and a cavity wall beyond the additional movable member remote from the injection volume, the gap being obscured from the injection volume when the additional movable part is advanced, the method including the steps of:
injecting a plastics material mixture comprising a basic polymer and a foam producing additive into the injection volume,
withdrawing the additional movable part after partial solidification of the plastics material mixture to form a skin against this part,
allowing the plastics material against the additional movable part to expand by foaming across the void including the gap, whereby the skin forms against the cavity wall.
The tool of the first embodiment has a further use, namely that which I originally envisaged, namely forming thick wall portions of foamed articles with a skinned outer wall.
Thus according to a third aspect of the invention there is provided a method of forming an article via injection of plastics materials into a mould, the method consisting in the steps of:
providing a mould tool defining in its closed state a mould cavity, the mould tool having a core part, a cavity part and an additional movable part whereby it is adapted to be partially expandable from an injection volume for receiving a load of plastics material to a final, larger volume determining the finish formed shape of the article, the additional movable part being adapted to be withdrawn to increase the volume of the mould cavity, the difference in the injection and final volumes being represented by a void exposed in the mould cavity when the movable part is withdrawn, the arrangement being such that there is a gap between the additional movable part when advanced and a cavity wall beyond the additional movable member remote from the injection volume, the gap being obscured from the injection volume when the additional movable part is advanced, the method including the steps of:
injecting a plastics material mixture comprising a basic polymer and a foam producing additive into the injection volume,
withdrawing the additional movable part after partial solidification of the plastics material mixture to form a skin against this part,
allowing the plastics material against the additional movable part to expand by foaming across the void including the gap, whereby the skin forms against the cavity wall.